It has long been appreciated that electric guitar amplifiers do more than make guitars louder. When driven to non-linearity or distortion, they add harmonic content that may improve the aesthetic appeal of the guitar sound. This is primarily the result of the limited ability of the amplifier to continuously track the input signal at high gain. As a result, the peaks of the signal waveform are rounded over or clipped off.
It has also been long appreciated that deliberately non-linear or distorting circuits may be connected in the signal chain between guitar and amplifier—“fuzz pedals” and the like—to obtain improvements in the aesthetic appeal of the guitar sound. Some distortion circuits are, in essence, amplifiers deliberately designed to operate in an overloaded state. Other distortion circuits use clipping diodes to create distortion. These circuits take advantage of the “diode drop” characteristic of diodes: A diode will not pass a substantial current in the forward direction until the forward voltage exceeds a certain minimum, called the forward voltage drop or “diode drop.” For silicon (Si) diodes, the diode drop is typically about 0.6 volts; for germanium (Ge) diodes, typically about 0.35 volts. Some diode distortion circuits include a clipping section that includes an anti-parallel pair of diodes connected between a signal conductor and ground. Any part of the signal that does not exceed plus or minus one diode drop is unchanged. However, when the signal exceeds plus or minus one diode drop, the diodes conduct to ground, clipping off the waveform peaks at plus or minus one diode drop. Other diode distortion circuits include a clipping section that includes an op amp and an anti-parallel pair of diodes in the feedback loop of the op amp, i.e., connected between the output and the inverting input. When the difference between the voltage at the output and the voltage at the inverting input exceeds plus or minus one diode drop, the diodes conduct, reducing the gain of the amplifier to one (in the non-inverting amplifier configuration) or zero (in the inverting amplifier configuration).
A number of U.S. patents have been issued purportedly disclosing various distortion circuits for use with musical instruments, including: U.S. Pat. No. 4,584,700; U.S. Pat. No. 4,021,613; U.S. Pat. No. 4,050,343; U.S. Pat. No. 4,180,707; U.S. Pat. No. 4,405,832; U.S. Pat. No. 4,811,401; U.S. Pat. No. 4,995,084; U.S. Pat. No. 5,022,305; U.S. Pat. No. 5,032,796; U.S. Pat. No. 5,133,014; U.S. Pat. No. 5,900,570; U.S. Pat. No. 5,977,474; U.S. Pat. No. 6,091,269.